Massive units like reactors have a fairly slow rate of cooldown from operational temperatures. In order to maintain such a unit safely, it must be cooled to a temperature that will allow maintenance workers to open and interact within the unit. Given the costs associated with downtime with systems like this, a need exists to cooldown units in a controlled accelerated manner.
Units have benefited from accelerated cooldown services. Typically this process is done in one of two ways. First, cool nitrogen gas can be passed through a unit. As the gas moves though the unit, it exchanges heat with any matter it comes into contact with, causing a faster than normal, or accelerated cooldown. In the alternative, cryogenic nitrogen fluid has been pumped into the gas stream within a specially designed system. The nitrogen is vaporized by the warm gas stream and forms mixed gas at a lower temperature. This cool gas mixture is used in the same manner as the gaseous cooldown to accelerate the cooling of the system.
In order to create the cool gas required for a gaseous cooldown, the cryogenic liquid nitrogen is vaporized and heated to a temperature that can be tolerated by the metallurgy of the system in question. The efficiency of a liquid cooldown is higher, because the energy to vaporize and heat up the gas from an extremely cold temperature are extracted from the system and not injected by the nitrogen equipment. As a general rule a cooldown with liquid is about 3.5 times more efficient than a gas cooldown. As a result it costs less than about 30% to cooldown a system with liquid as compared to gas.
There are several limitations with liquid nitrogen cooldown that restrict its application within industry. The metallurgy of the system must be compatible with cryogenic temperatures. Pipes made from stainless steel with high nickel content cannot tolerate liquid nitrogen temperature. Moreover, the system must have a carrier gas in order to vaporize and carry the gas mixture throughout the system. Furthermore, a system that recycles its gas can more fully utilize the cooling power of the liquid. Finally, cryogenic nitrogen liquid will destroy most reactor systems.
There are also limitations on gas cooldown methods. The limiting factor in gas cooldown methods is the amount of product required to cool down any substantially large system. It is the transport of the liquid to site that is more of a factor than the bulk cost of the nitrogen. This creates an effective radius of application. Beyond this radius, while accelerating the cooling of a reactor is attractive, the costs of doing the operation out weigh the benefits in all but the most extreme situations. Therefore, a need exists to accelerate the cooldown of systems and units using a liquid medium that does not require the application of expensive cryogenic piping in a method that will not damage the carbon steel of these systems.
The prior art has only used carbon dioxide that was actually injected right into the reactor to control the temperature of an exothermic reaction. Direct injection into a reactor or similar vessel does not produce good flow characteristics during shutdown. Without even distribution of a cooldown medium, the cooldown of the reactor will take longer. There exists a need to be able to take advantage of the open space, preferably with a high velocity gas, by putting it into the feed pipe of the reactor or into the combustion air intake airflow to a boiler furnace. Moreover, a need still exists for a system and a method of its use that will allow for using existing piping to provide for a well distributed cooling method and to accelerate the cooldown of a unit during downtime and maintenance rather than attempting to control the reaction itself. The prior art has failed to offer an efficient and safe manner of accelerating the cooldown of a unit so that the system will be safe to enter as quickly as possible.